The exploration for and production of hydrocarbons such as oil and gas requires the drilling of wellbores to locate and produce the hydrocarbons. Subsequently, the wellbores are completed so that the hydrocarbons can be produced to the surface. Historically, large diameter boreholes were drilled to access the reservoirs. Because the volume of the wellbore cylinder is equal to a dimension defined as pi multiplied by the cylinder length and by the cylinder radius squared, the diameter of a wellbore significantly affects the quantity of rock cuttings that must be removed. For example, a reduction in the wellbore diameter by one-half reduces the quantity of rock cuttings by three-quarters. Because the cost of a wellbore is directly proportional to the quantity of rock cuttings removed, there is a need to reduce the size of the wellbore and associated equipment necessary to drill and produce the wellbore.
Small diameter wellbores, created by "slimhole" drilling techniques, are being used in certain applications as a vehicle for reducing drilling and production costs. The smaller wellbores utilized downsized drilling rigs, less manpower, and fewer supplies and equipment. The reduction in equipment significantly reduces the costs associated with transporting the drilling rig into remote locations by helicopter. Slimhole wellbores are also used in production wells to establish multiple horizontal wells from an existing vertical well, and to extend the depth of existing vertical wells. Slimhole wellbores are used in exploration programs to reduce the cost of obtaining geological and geophysical data in remote and unexplored locations. For example, slimhole stratigraphic tests such as coring programs can supplement seismic data so that a geochemical, lithological, and petrophysical model of the basin and hydrocarbon bearing reservoirs can be developed.
Coring operations physically remove a core sample of the rock from the wellbore so that the core sample can be evaluated. The core data provides geological and geoscientific data regarding the potential of a geologic basin, including information regarding the content of source material and the requisite temperature necessary to form hydrocarbons from the source material. A coring drill bit is rotated to encapsulate a cylindrical core sample within a core barrel, and the core barrel is removed to the well surface with a wireline or pumping techniques known in the art. Continuous coring operations use a retrievable core barrel that can be pulled through the drill string with a wireline without removing the drill bit from the bottom of the wellbore.
The viability of a slimhole drilling program is significantly affected by well safety concerns. When a wellbore is drilled through a reservoir containing hydrocarbons under pressure, gas in the reservoir can enter the drilling column. As the pressure on the gas is released, the gas will expand and displace the drilling fluid in the drill string. These gas "kicks" are usually controlled by the weight of drilling fluid circulated through the wellbore during drilling operations. A slimhole drilling rig requires only fifty to three hundred barrels of mud, which is substantially less than a conventional drilling system. Because slimhole wells have a smaller diameter that substantially reduces the volume of drilling mud available in the well to control gas pressure, well control is a significant concern in slimhole drilling operations. Although slimhole drilling systems have been developed to closely monitor the wellbore during drilling operations, the circulation of drilling mud through the drill string is a critical factor in maintaining safe drilling operations.
The removal of a core barrel from the drill string can encumber the circulation of drilling mud necessary to maintain control of the well. As the core barrel is run into and out of the drill string during drilling operations, the core barrel displaces fluid and creates swabbing pressures within the drill string. Running out of the drill string too quickly can reduce the hydrostatic head and lead to an influx of hydrocarbon fluids. Running into the drill string too quickly can exceed the formation fracture gradient and cause lost circulation. These problems are particularly evident at the tool joint connecting two sections of drill pipe. The interior bore of the pipe ends at the tool joint is typically smaller than the bore of the pipe mid section. This smaller internal diameter limits the circulation of drilling fluid as the core sample passes through the tool joint, and interferes with the pressure of the fluid necessary for pressure control of the well. Accordingly, a need exists for an apparatus and method that can permit coring operations in slimhole drilling while pressure control of the well is maintained.